COMPARING SR ISOTOPIC TECHNIQUES TO ASSESS THE DEGREE OF ALTERATION IN SHELLS USED FOR STRONTIUM ISOTOPE STRATIGRAPHY

Accurate estimates of the age of ancient shorelines associated with periods of warm-climate are an essential component in predicting current sea level change. One common approach in dating ancient shoreline deposits is the use of Strontium Isotope Stratigraphy (SIS). A vital part of SIS is appraisal of the preservation of the original ⁸⁷Sr/⁸⁶Sr ratio, because calcium carbonate shells are susceptible alteration during diagenesis, which can shift the ⁸⁷Sr/⁸⁶Sr ratio leading to inaccurate ages. Yet assessing SIS samples for possible alteration remains problematic. Here we report on a series of experiments performed to explore the potential for using isotopically based techniques to screen SIS samples for alteration. Three techniques were assessed using shells that displayed varying degrees of preservation of their original ⁸⁷Sr/⁸⁶Sr ratio, including; 1) leaching of powdered shells, 2) leaching of whole shells, 3) sequential sampling of shell material using a microdrill.

Results from analysis of successive leaches on powdered shells indicate that alteration produces a characteristic pattern where the ⁸⁷Sr/⁸⁶Sr ratio of the first leach is higher than subsequent leaches. Leaching of powders may therefore remove easily soluble contaminating Sr from a sample, however the degree to which any contamination may be scrubbed remains un-quantified. Microdrilling from the edge to the center of shells produces similar patterns, characterized by ⁸⁷Sr/⁸⁶Sr ratios in the outermost layers that are higher than the inner portions of the shell, suggesting that a reaction front is present in partially altered shelly material. Sequential leaches performed on whole shells produced no characteristic pattern, indicating that this method is not useful in either identifying or scrubbing contamination. Our initial results suggest poorly preserved samples are contaminated by Sr with a ⁸⁷Sr/⁸⁶Sr ratio higher than the original Sr seawater composition; and that contamination may be localized to the outer edges of shells. We note a strong correlation between measured ⁸⁷Sr/⁸⁶Sr values in fully dissolved shells and the calculated variance of the ⁸⁷Sr/⁸⁶Sr ratio of successive leaches/microdrill aliquots. We suggest that the level of variance seen within a sample allows us to identify whether or not the shell will provide an accurate SIS age.